Cold cathode fluorescent lamp (CCFL), external electrode fluorescent lamp (EEFL), and other types of discharge lamps are widely used to backlight liquid crystal displays (LCD). Such discharge lamps all require a driving mechanism for supplying an alternating current (AC) driving voltage and a stable high-frequency lamp current.
Typically, discharge lamps require a striking voltage (e.g., a few hundred volts) to initiate or strike an electrical arc in the discharge lamps. The striking voltage can be even higher (e.g., 1000-2000 volts) under low temperature and/or aging conditions. Once an electrical arc is struck inside the discharge lamps, the terminal voltage may fall to an operation voltage (e.g., a few hundred volts), and the brightness produced depends on the current flowing through the discharge lamps. When a driving circuit detects that a discharge lamp is in an open circuit (e.g., the electric arc has not been struck yet; the lamp is not properly coupled to the terminals; or the lamp malfunctions), the driving circuit would provide the striking voltage to the terminals and attempts to re-strike the electric arc in the discharge lamp. If the driving circuit still detects an open circuit after a preset amount of time, the driving circuit would determine that the lamp is not properly coupled to the terminal or the lamp has malfunctioned, and cease attempting to re-strike the electric arc in the discharge lamp for self-protection.
Typically, conventional driving circuits adjust the brightness of discharge lamps based on a lamp current feedback signal in normal operation, and adjust the terminal voltage based on a lamp voltage feedback signal in open circuit conditions. The driving circuits can also include protection circuitry that monitors the terminal voltage and terminates the driving circuits when the terminal voltage exceeds a threshold for longer than a preset amount of time (e.g., 1 second). To provide a sufficient striking voltage, the driving circuits often utilize frequency hopping techniques in which the working frequency is increased to a preset value after an open circuit is detected.
FIG. 1 is a block diagram of a driving circuit for driving a single discharge lamp in accordance with the prior art. As shown in FIG. 1, the driving circuit includes a switching circuit 101, a control circuit 102, a transformer 103, a resonant circuit 104, and a load 105 that includes a single discharge lamp L. The switching circuit 101 comprises at least one switch that receives a direct current (DC) input voltage Vin and generates a switching signal SW. The control circuit 102 is electrically coupled to the switching circuit 101 and controls the on/off of the at least one switch. The transformer 103 is electrically coupled between the switching circuit 101 and the resonant circuit 104. The primary winding of the transformer 103 receives the switching signal SW, and the secondary winding of the transformer 103 accordingly generates an AC signal. The resonant circuit 104 is electrically coupled between the transformer 103 and the load 105. The resonant circuit 104 receives the AC signal and generates an output voltage Vout to drive the load 105.
When the input voltage Vin and circuit parameters are constant, the output voltage Vout of the driving circuit is determined by the duty cycle of the switching signal SW and the voltage gain of the resonant circuit 104 and the load 105. The voltage gain is related to the operating conditions of the load 105 (whether the lamp L is open) and the switching frequency of the switching signal SW. Typically, the lamp current or the lamp voltage is monitored and compared with a threshold to detect whether the lamp is under open circuit condition. However, in a transient open circuit state, the duty cycle of the switching signal does not have time to adjust, and there is a delay between the lamp reaching open circuit and the driving circuit detecting the open circuit condition.
FIG. 2 is a curve showing a relationship between the switching frequency and the voltage gain of the resonant circuit 104 and the load 105 as a function of the switching frequency. In normal operation, the gain curve is curve a, the switching frequency is the operation frequency fs, and the voltage gain is G1. The corresponding output voltage Vout is the normal working voltage Vo,normal. The operation frequency fs is generally set to be slightly higher than the resonant frequency of the resonant circuit 104 and the load 105. Under open circuit conditions, the gain curve is the curve b. If the switching frequency is maintained at the operation frequency fs, the voltage gain will be G2>G1. The difference between G2 and G1 is determined by the resonant parameters of the resonant circuit 104 and also the characteristic of the lamp L.
Generally, G2 is not large enough to allow the output voltage Vout to reach the striking voltage, so a frequency hopping technique is usually used. Once the open circuit condition is detected, the switching frequency is set to a higher frequency fs,open to obtain a voltage gain G3, and G3>G1,G2. The frequency fs,open may be set by external resistors or voltages, or it may be set internally. If the frequency fs.open is set internally, under some conditions (related to the resonant parameters of the resonant circuit 104), the instant output voltage Vout during frequency hopping may be too high to cause the failure of the lamp L and/or the other electrical elements.
FIG. 3 is a diagram illustrating a waveform of the peak output voltage Vout with respect to time during lamp initiation. At t=0, the driving circuit is powered on, the lamp L is not ignited, and the open circuit condition is not detected. The switching frequency is the operation frequency fs and the voltage gain is G2. During 0<t<t1, the duty cycle of the switching signal SW is increased by the control circuit 102, and the output voltage Vout is increased accordingly. At t=t1, the open circuit condition is detected, the frequency is set to the frequency fs,open, and the voltage gain is G3. If G3 is large enough, there will be an overshoot Vos1 across the lamp L. Then, the duty cycle of the switching signal SW is decreased by the control circuit 102 until the output voltage Vout is regulated to the striking voltage Vo,strike. At t=t2, the lamp L is ignited, the switching frequency is set to be the operation frequency fs again, the voltage gain is G1 and the output voltage Vout is the operation voltage Vo,normal.
FIG. 4 is a diagram illustrating a waveform of the peak output voltage Vout with respect to time before and after a lamp opening. Before t=t3, the driving circuit is in normal operation, the switching frequency is the operation frequency fs, the voltage gain is G1 and the output voltage is the operation voltage Vo,normal. At t=t3, the lamp L is open, but the open circuit condition is not detected, the switching frequency is maintained at the operation frequency fs, the voltage gain is G2 and the output voltage is Vos2. During t3<t<t4, the duty cycle of the switching signal SW is increased by the control circuit 102, and the output voltage Vout is increased accordingly. At t=t4, the open circuit condition is detected, the frequency is set to the frequency fs,open, and the voltage gain is G3. If the difference between G3 and G2 is large enough, there will be an overshoot Vos3 across the lamp L. Then, the duty cycle of the switching signal SW is decreased by the control circuit 102, until the output voltage Vout is regulated to the striking voltage Vo,strike. At t=t5, the lamp L is ignited again, the switching frequency is set to be the operation frequency fs, the voltage gain is G1 and the output voltage Vout is the operation voltage Vo,normal.
FIG. 5 is a block diagram of a driving circuit for driving two serially connected discharge lamps. The driving circuit is similar to the one shown in FIG. 1, except that the load 105 comprises two serially connected discharge lamps L1 and L2. The lamps L1 and L2 may not be ignited at the same time because of their characteristic differences. If L1 is ignited first, its instant impedance will be decreased during ignition, which will cause an overshoot across L2. L1 may be ignited before or after the open circuit condition is detected. If the frequency hopping technique is used, there will be two overshoots across L2, one caused by the frequency hopping, and the other caused by the ignition of L1. If one of the two discharge lamps opens during normal operation (e.g., L2 is open), its instant impedance will increase during circuit opening to cause a voltage overshoot across L2. After a delay, if the driving circuit detects the opening circuit condition and uses the frequency hopping technique, it will cause another overshoot across L2.